1. Field of the Invention
The present invention relates to testing radar systems. More specifically, the present invention relates to a compact range for testing radar systems.
2. Discussion of the Related Art
One of the final steps in testing and qualifying airborne radar is to flight test it. Once a minimum set of fight tests is performed and the radar is deemed working, a specified burn-in is flown to mitigate problems associated with infant mortality. The flight in the aircraft applies motion, vibration, realistic flight temperatures and altitudes, and other environmental characteristics relevant to the real world operation of the radar. Flight testing is considered to be the final arbiter of a good radar system. Flight testing is especially important for synthetic aperture radar (SAR). These types of radars require precise knowledge of the motion of the aircraft to form the imagery they produce. Because of limited access to airplanes and the high cost of testing radars on aircraft, however, there is a need to find a substitute for some aspects of the flight test and burn-in when a radar system needs to be tested.
Large outdoor ranges have been envisioned in order to simulate the movement of radar. SAR imagery relies on relative motion between the radar and the target and refers to the situation where the radar moves and the target is fixed. When the target moves and the radar stays fixed, the radar is termed an inverse synthetic aperture radar (ISAR). In order to simulate movement of the radar using ISAR, one or more targets are typically placed on a rotating table and with the radar stationary. The radar thus makes an image as if it were flying around the targets in a circular orbit.
The problem with the ISAR scheme is that the minimum range at which the radar can work is often very large. These long minimum ranges are determined by the time the radar takes to receive the pulse after it has transmitted it. Due to pulse risetimes and other timing considerations, this minimum range is about 500 meters in a typical radar system. Additionally, there may be other difficulties with such a rapid switching in other parts of the radar circuitry, notably in the digital waveform synthesizer. Moreover, working at such minimum ranges is very different from the normal operation of the radar, where ranges of the order of 4 km to 80 km or more are typical.
Such large minimum ranges mean that the ISAR range usually needs to be placed outdoors. Siting such a range can be quite a challenge in a business park environment where trees and buildings make it very difficult to find an adequately long line of sight distance that is also safe from the point of view of human radio frequency exposure. Rooftop to rooftop systems may be envisioned, but these can also be problematical. It may be hard to find buildings that are sufficiently stable mechanically to support a turntable on which to move the targets the radar will image. In addition, outdoor turntable systems are susceptible to uncompensated motion due to wind.
Therefore, a system which can test a radar system without the need for an aircraft or a large outdoor range is needed in order to overcome the above mentioned problems.